Although cell-mediated immune responses to a variety of alloantigenic stimuli have been studied thoroughly in the past, responses that occur after prior alloantigenic stimulation have not been previously thoroughly investigated. Our studies involve an assessment of cell-mediated immune responses after the host has responded to a variety of alloantigenic stimulation to the same and other antigens. We find a marked specific decrease in the generation of cytotoxic cells after previous immunizations. The diminished response is specific for the immunizing alloantigen. Passively transferred high titered alloantisera causes some decrease in cytotoxic cell generation but thus far no decrease occurs following passively transferred cells. Present efforts involve further definition of humoral and cellular mechanisms of the depressed cytotoxicity generated in previously immunized animals. The significance of this work comes in attempts to generate large numbers of specifically programmed cytotoxic cells for adoptive transfer to immunity to tumors.